Walter Jahn does his bottom fishing with 5,000-pound-test line, which is sporting enough considering that he sometimes trolls as deep as 30,000 feet.

What Jahn goes after is not fish but photographs of the ocean floor, an area far larger and less well known to us than the dark side of the moon. Part of his splendid catch goes on display this weekend at the Smithsonian Museum of Natural History.

The 24 photographs, enlarged to mural size, show scenes of the two-mile-high Gilliss Seamount, one of a mighty range of volcanic mountains that marches 1,200 miles eastward across the Atlantic floor from off Cape Cod. These abyssal Alps were unknown until 1955 because the tip of the tallest is nearly a mile below the surface. The Navy kept the discovery secret for several years because of the implications for undersea warfare, and still tells less than it knows.

The topography of the range is still being laboriously charted by sonar, but a sense of the deep-seascape depends upon human- scale scenes such as those Jahn has brought us. He used a 620-pound camera rig he designed "to photograph from much the same perspective as you would if you were standing there," he said. Most such pictures used to be taken from directly overhead, which suppresses the relief and tends to wash out the foreground.

Since the rig dangles from the end of a 1/4-inch wire cable, Jahn never knows what he has until he develops his film. "We know the precise latitude, longitude and depth, and aim for particular areas on sonar maps," he said, "but what with crosscurrents bellying the cable and the roughness of the landscape, it's pretty much hit or miss." The cameras are fired when a dangling weight touches bottom. What is lost in camera control is more than made up in economy, since deep-diving submersibles cost something like their weight in gold.

Jahn, an oceanographer with the Naval Ocean Research and Development Activity, took the black-and-white Gilliss photographs during a 1974 cruise of the Navy research vessel Lynch. Since high-speed Ektachrome has become available he has gone over to color photography, "which gives you much more information per frame."

Perhaps so, but the faithful old Tri-X he used on Gilliss gives more of the eerie feel of such awesome depths than anything I've ever seen from Jacques Cousteau.

DEEP OCEAN PHOTOGRAPHY -- At the Museum of Natural History through April 19.